A push button switch of Patent Document 1 in which an operating part and a switch part are configured to be able to separate from each other is used as a switch for commanding especially a machine tool to stop in an emergency. As shown in FIG. 6, such a push button switch is configured by a switch operating part 1 and a switch part 2 attached to the operating part 1 in such a manner as to be separable therefrom, and is attached to a panel such as a control board for use.
Procedures for attaching the push button switch to a panel are described hereinafter.
The operating part 1, from which the switch part 2 is removed, has a body 11 inserted from the front side of the panel to a mounting hole thereof, and then a fixing nut 19 is screwed from the back side of the panel into the inserted body 11 of the operating part 1 and tightened, thereby fixing the operating part 1 to the panel. A fitting hole 21a of the switch part 2 is fitted to the body 11 of the operating part 1 attached to the panel, to couple and join the switch part 2 to the operating part 1.
In some cases, the push button switch that is attached to a control board or the like as described above might get involved in an accident in which the switch part 2 falls off of the operating part 1 for some reason while the push button switch is in use. When the switch part 2 is removed from the operating part 1 during a push operation of the push button switch, the pressed state of the switch part 2 accomplished by a push stick of the operating part 1 is canceled, and consequently a movable frame that retains a movable contact of the switch part 2 is returned to its return position by a return spring, returning a normally open contact unit to an OFF state and a normally closed contact unit to an ON state.
In a case in which the normally closed contact unit is in use in order to issue an emergency stop instruction, the occurrence of such accident where the switch part falls off from the operating part returns the normally closed contact unit to its ON state and thereby cancels the emergency stop instruction. As a result, the operation of the equipment that has stopped for an emergency might be resumed, causing a serious accident.
In order to avoid such danger, a released movable contact of an open/close contact unit having a normally closed contact configuration needs to be prevented from returning to a closed state along with a fixed contact when the switch part 2 of the push button switch is removed from the operating part 1, the open/close contact being used especially as an open/close contact unit for issuing an emergency stop instruction.
Therefore, the push button switch described in Patent Document 1 is provided with, as shown in FIGS. 7 and 8, a contact releasing mechanism 3, which is configured by a small lever 31 that has one end supported rotatably in the switch part 2 of the push button switch and acting on a part of the operating part 1 joined to the switch part 2 and has the other end acting on a movable frame 25 of an open/close contact unit of the switch part 2 to open/close the contacts, and a spring 33 for urging the lever 31 in a certain rotational direction, an open/close contact mechanism 24c, and a unit frame 24f. When the switch part 2 is separated from the operating part 1 by the contact releasing mechanism 3, the movable frame 25 of the open/close contact unit 24 is urged so as to move a movable contact from a standby state position to an operating state position, and when the switch part is joined to the operating part the urging operation of the contact unit performed on the movable frame 25 is canceled.
The small lever 31 used in the contact separating mechanism of the push button switch and the open/close contact mechanism (base member) 24c of the switch part 2 that rotatably supports the small lever 31, are made of resin. In order to rotatably support the lever 31 on the open/close contact mechanism 24c, a columnar support shaft 31c is formed in a protruding manner on an outer surface of the lever 31, and a bearing groove or a hole 24e is provided in the open/close contact mechanism 24c. The support shaft 31c of the lever 31 is fitted into the bearing groove or hole 24e of the open/close contact mechanism 24c. 
Patent Document 1: Japanese Patent Application Publication No. 2010-232157